I. Field of the Invention
The present invention relates to a method for packaging electronic parts such as IC chips mounted on a multilayer substrate. More particularly, the present invention relates to a method for hermetically sealing or packaging such electronic parts by laser welding within a space defined by the multilayer substrate and a cap covering it.
II. Description of the Prior Art
The demand for compact, light-weight, and highly reliable electronic equipment has been steadily increasing. In order to meet this demand, a multichip packaging technique has been developed wherein a number of electronic parts such as IC chips are mounted on a high-density multilayer substrate of alumina ceramic or the like and the entire structure is hermetically sealed. Conventional hermetic sealing techniques include encapsulation within a plastic molding by injecting a melted epoxy resin onto the multilayer substrate. However, plastic molding has the disadvantage of low humidity resistance and also does not allow preparation of a highly reliable product. In order to improve the humidity resistance, a hermetic sealing technique has been developed using a solder or low-melting point glass. However, the product of this technique has a low heat resistance.
At present, it is considered that the most reliable hermetic sealing technique is sealing by welding. According to this method, a ring-shaped welding rib is formed around an alumina ceramic multi-layer substrate on which electronic parts such as IC chips have not yet been mounted. The welding rib is made of an Fe/Ni/Co alloy or Fe/Ni alloy which has substantially the same thermal expansion coefficient as that of the substrate. A thin metal film such as an Au film is formed by electrolytic or nonelectrolytic plating to cover the entire surface of the conductor portions and the welding rib on the board so as to achieve good wire bonding characteristics of the electronic parts and to improve corrosion resistance. Then, electronic parts are mounted on some of the conductor portions of the multilayer substrate through an adhesive such as conductive epoxy resin. Subsequently, a cap is placed on the welding rib. The cap is made of the same Fe/Ni/Co alloy or Fe/Ni alloy which is used for the welding rib. The outermost shape of the cap and its measurements are substantially the same as those of the welding rib. The periphery of the cap is welded to the welding rib, thereby hermetically sealing the electronic parts within a space defined by the substrate and the cap. Welding is performed by seam welding or by laser welding.
However, seam welding has a relatively low welding power. A large cap is required when a large product is manufactured. When the large cap is used, a cap thickness must be increased to a range between 0.15 mm and 0.5 mm so as to obtain proper mechanical strength. However, since seam welding has a low welding power, it is difficult to weld such a thick cap to a corresponding welding rib. For this reason, laser welding is used when such a cap having a thickness of 0.15 to 0.5 mm is used.
As described above, the thin metal film such as an Au film is formed to cover the entire conductor portions of the multilayer substrate so as to improve wire bonding characteristics and to prevent corrosion. The thin metal film is formed by electrolytic or nonelectrolytic plating. In a product having a high packaging density, a number of floating conductors which are not connected to the external part of the board are present. When the number of floating conductors increases, it is very difficult to electrolytically plate all the floating conductors. This is because each electrode must be assigned to each floating conductor. For this reason, the thin metal film must be formed by nonelectrolytic plating in a highly integrated product.
More specifically, in the case of manufacturing a large product which has a high packaging density, a thin metal film is formed by nonelectrolytic plating on conductor portions of a multilayer substrate, and the cap periphery is welded by laser welding to the welding rib. Nonelectrolytic plating is performed by dipping the multilayer substrate which has the welding rib thereon in a nonelectrolytic plating solution. Therefore, the thin metal film is formed on the welding rib. As a result, this film is present in welding, inbetween the cap periphery and the welding rib. However, a resultant welded portion may crack over time, thereby degrading the hermetic condition of the product, and hence, its reliability.